In recent electricity generating installations, for high power generation efficiency and effective energy use, there has been increasing use of gas turbines using a high temperature combustion gas such as liquefied natural gas (LNG) or combined cycle generating installations in which a gas turbine and a steam turbine are combined. In these electricity generating installations, with the increasingly high combustion gas temperatures, the thermal load on the turbine oils used has greatly increased.
Further, in the case of combined cycle electricity generation burning blast furnace gas (BFG), because the combustion calories from BFG are low, it is necessary greatly to increase the pressure of the BFG in order to increase the power generation efficiency. Because of this, the BFG is fed into the gas turbine after compression in a gas compressor connected with the shaft system made up of gas turbine-electricity generator-steam turbine via a multiplier gear.
In this multiplier gear, the gas compressor and turbine shaft and electricity generator are directly connected; however, to make the turbine plant more compact, it has become a requirement that the turbine bearing lubricating oil and the multiplier gear lubricating oil can be used for both purposes. For such a lubricating oil, performance both as a turbine oil and as a gear oil is required, and gear antiwear properties and extreme pressure properties are strongly required. Moreover, excellent rust prevention properties are required, and it is also required to have excellent thermal and oxidation stability and anti-sludge performance under severe high temperature and high surface pressure conditions.
Because of this, gas turbine oils have previously been proposed (see fore example Japanese Laid-Open Specification No. 7-228882) wherein an alkylated diphenylamine, alkylated phenyl-a-naphthylamine and benzotriazole are incorporated into a mineral oil or synthetic oil; however, satisfactory effects had not yet been obtained with these.
As stated above, when previous lubricating oils were used in the turbine bearings of combined cycle generators having an aforesaid multiplier gear, their anti-sludge performance and extreme pressure properties could not always be described as satisfactory. That is to say, in applications where excellent rust prevention properties, excellent extreme pressure properties and antiwear properties were required, lubricating oils to which sulphur type extreme pressure additives such as zinc dialkyldithiophosphates or sulphur-phosphorus type extreme pressure additives such as alkylated thiophosphates, and, as rust prevention agents, Ca sulphonates or Ba sulphonates and the like, are added are widely used; however, since by their nature the rust prevention agents have excellent adsorption onto metal surfaces, there is a high probability that they will impair the lubricating performance-improving action of the various extreme pressure additives, hence it is very difficult to reconcile extreme pressure properties with rust prevention properties.
In particular, in applications where lubricating agents are used at high temperature, even when the quantity of sulphur type extreme pressure agent added is extremely small, large amounts of sludge tend to be formed if the thermal load is increased, and the thermal stability and oxidation stability tend to decrease. Because of this, with lubricating oils with added sulphur type extreme pressure agents it is difficult to obtain satisfactory thermal stability, oxidation stability and anti-sludge performance in the turbine bearings of combined cycle generators having the aforesaid multiplier gears.
On the other hand, although less sludge tends to form with the phosphorus type extreme pressure additives than with the sulphur type extreme pressure additives, it is difficult to obtain the high level extreme pressure properties required in the aforesaid gear bearings if the phosphorus type extreme pressure additives are used alone.